A thousand Best Buy stores across the nation began stocking Green Your Phone cards. Developed by Boulder-based Renewable Choice Energy, each $10 card will offset the electricity use of two cell phones for two years, the company says.

The cards are loaded with 500 kilowatt hours of certified renewable energy credits, and purchases support the development of renewable energy projects such as wind power farms in Kansas, Idaho, Wyoming and Minnesota, according to the Boulder company.

The credits are verified by Green-e to ensure that no two credits represent the same electricity put on the grid. Since the purchase of credits is voluntary by customers, they are not regulated by state utility commissions.